1. Field of the Invention
The invention relates to LZ dictionary based data compression systems particularly with respect to the LZW compression methodology. More particularly, the invention relates to the architecture and protocols for storing and accessing data character strings in the compressor in parallel arrangements.
2. Description of the Prior Art
Professors Abraham Lempel and Jacob Ziv provided the theoretical basis for LZ data compression and decompression systems that are in present day widespread usage. Two of their seminal papers appear in the IEEE Transactions on Information Theory, IT-23-3, May 1977, pp. 337-343 and in the IEEE Transactions on Information Theory, IT-24-5, September 1978, pp. 530-536. A ubiquitously used data compression and decompression system known as LZW, adopted as the standard for V.42 bis modem compression and decompression, is described in U.S. Pat. No. 4,558,302 by Welch, issued Dec. 10, 1985. LZW has been adopted as the compression and decompression standard used in the GIF image communication protocol and is utilized in the TIFF image communication protocol. GIF is a development of CompuServe Incorporated and the name GIF is a Service Mark thereof. A reference to the GIF specification is found in GRAPHICS INTERCHANGE FORMAT, Version 89a, Jul. 31, 1990. TIFF is a development of Aldus Corporation and the name TIFF is a Trademark thereof. Reference to the TIFF specification is found in TIFF, Revision 6.0, Finalxe2x80x94Jun. 3, 1992.
Further examples of LZ dictionary based compression and decompression systems are described in the following U.S. patents: U.S. Pat. No. 4,464,650 by Eastman et al., issued Aug. 7, 1984; U.S. Pat. No. 4,814,746 by Miller et al., issued Mar. 21, 1989; U.S. Pat. No. 4,876,541 by Storer, issued Oct. 24, 1989; U.S. Pat. No. 5,153,591 by Clark, issued Oct. 6, 1992; U.S. Pat. No. 5,373,290 by Lempel et al., issued Dec. 13, 1994; U.S. Pat. No. 5,838,264 by Cooper, issued Nov. 17, 1998; U.S. Pat. No. 5,861,827 by Welch et al., issued Jan. 19, 1999; and U.S. Pat. No. 6,188,333 by Cooper, issued Feb. 13, 2001.
In dictionary based LZ compression and decompression systems, the compressor and decompressor dictionaries may be initialized with all of the single character strings of the character alphabet. In some implementations, the single character strings are considered as recognized although not explicitly stored. In such systems the value of the single character may be utilized as its code and the first available code utilized for multiple character strings would have a value greater than the single character values. In this way the decompressor can distinguish between a single character string and a multiple character string and recover the characters thereof. For example, in the ASCII environment, the alphabet has an 8 bit character size supporting an alphabet of 256 characters. Thus, the characters have values of 0-255. The first available multiple character string code can, for example, be 258 where the codes 256 and 257 are utilized as control codes as is well known.
In the prior art dictionary based LZ compression systems, data character strings are stored and accessed in the compressor dictionary utilizing well known search tree architectures and protocols. Typically, the search for the longest matching string stored in the dictionary is an iterative process where sequentially matched strings in the dictionary are extended by sequentially fetched input characters, respectively, until the longest matching string is determined. At each iteration, the dictionary is accessed to determine if the new string extension is a previously stored dictionary entry. Potentially, at each iteration, access to all of the strings stored in the dictionary may be effected to determine the required information. For example, in systems implemented utilizing an associative memory dictionary, such as in said U.S. Pat. Nos. 5,373,290 and 5,838,264, it may be necessary, at an iteration, to access all dictionary locations to determine that an extended string is noted stored therein. The iterative protocols, therefore, tend to be time consuming.
The searchtree is typically arranged in nodes where each node represents a character, and a string of characters is represented by a node-to-node path through the tree. When the input character stream has been matched in the dictionary tree up to a matched node, a next input character is fetched to determine if the string match will continue. Conventionally, a determination is made to ascertain if the fetched character is already stored as an extension node of the matched node. Various techniques are utilized to effect this determination such as hashing and sibling lists as are well understood in the art.
Although the known dictionary architectures and protocols provide efficient data compression systems, it is a continuing objective in the art to improve compressor performance.
The compressor of said Ser. No. 09/855,127 utilizes a new string storage and access architecture and protocols involving character tables which, it is believed, will improve the performance of LZ type data compression algorithms. The compressors of said Ser. No. 09/951,263 utilizes a new string storage and access architecture and protocols involving cascaded subdictionaries which, it is believed, will improve the speed of dictionary based LZ type data compressors. It is a further objective of the present invention to combine the technologies of said Ser. No. 09/855,127 and Ser. No. 09/951,263 to synergistically obtain the advantages of both.
The present invention provides a new string storage and access architecture and protocols which, it is believed, will further improve the performance of dictionary based LZ type data compressors.
In the present invention, a plurality of character tables, corresponding to respective characters of the alphabet, are arranged in levels for storing strings of data characters encountered in the input stream. The strings stored in the character tables of a level have the same number of characters with respect to each other and the strings stored in the character tables of a level have one character more than the strings stored in the character tables of the level prior thereto. A string is stored in a character table by storing the string code associated with the string in a character table corresponding to the extension character of the string at a character table location corresponding to the code of the string prefix. A plurality of data characters are fetched from the input and applied to the levels, respectively. The fetched characters are searched by comparing the fetched characters to the stored strings to determine the longest match therewith. The longest match is determined by one of the fetched characters resulting in a mismatch at one of the levels. The string code associated with the longest match is output so as to provide the output stream of compressed codes corresponding to the input stream of data characters. An extended string comprising the longest match extended by the fetched character that resulted in the mismatch is stored at the mismatching level in the character table corresponding to the mismatching character.